White Sands National Monument, New Mexico, United States

Description
Gypsum is rarely found in the form of sand because it is water-soluble. Normally, rain would dissolve the gypsum and carry it to the sea. The Tularosa Basin is enclosed, meaning that it has no outlet to the sea and that rain that dissolves gypsum from the surrounding San Andres and Sacramento Mountains is trapped within the basin. Thus water either sinks into the ground or forms shallow pools which subsequently dry out and leave gypsum in a crystalline form, called selenite, on the surface. Groundwater that does flow out of the Tularosa Basin flows south into the Hueco Basin. During the last ice age, a lake known as Lake Otero covered much of the basin. When it dried out, it left a large flat area of selenite crystals which is now the Alkali Flat. Another lake, Lake Lucero, at the southwest corner of the park, is a dry lake bed, at one of the lowest points of the basin, which occasionally fills with water.
The ground in the Alkali Flat and along Lake Lucero's shore is covered with selenite crystals which reach lengths of up to three feet (1 m). Weathering and erosion eventually breaks the crystals into sand-size grains that are carried away by the prevailing winds from the southwest, forming white dunes. The dunes constantly change shape and slowly move downwind. Since gypsum is water-soluble, the sand that composes the dunes may dissolve and cement together after rain, forming a layer of sand that is more solid and could affect wind resistance of dunes. This resistance does not prevent dunes from quickly covering the plants in their path. Some species of plants, however, can grow fast enough to avoid being buried by the dunes.